1. Field of the Invention
The present invention relates to a transmitting/receiving device and a transmitting/receiving method for wireless-transmitting/receiving analog acoustic signals.
2. Description of the Related Art
Conventionally, an infrared transmitting/receiving device in which an analog acoustic signal is modulated and wireless-transmitted through a transmitting device by using infrared radiation or the like, the analog acoustic signal is demodulated by a receiving device, and the analog acoustic signal comes as a sound out of a speaker or the like is well-known. FIG. 9 is a system diagram of the above infrared transmitting/receiving device. In FIG. 9, a reference numerals designates a transmitting device as a first device and 2 a receiving device as a second device. Through a transmission line between the transmitting and receiving devices 1 and 2, transmission is carried out by using infrared radiation.
An analog acoustic signal such as voice and tone is supplied as an input signal to an input terminal T1 of the transmitting device 1.
The input signal is supplied to an emphasizer 3. In the emphasizer 3, a high-frequency boosting characteristic HB for boosting high frequencies of the input signal is applied to the input signal as shown in an emphasis characteristic diagram in FIG. 10(A) in which a level of the input signal is plotted in a vertical axis and a frequency of the input signal is plotted in a horizontal axis and the input signal is supplied to a compression circuit 4 for compressing the voice and the like in the next stage.
The compression circuit 4 is formed of: a line resistor R1; a time constant circuit 4a in which an anode of a diode CD is connected to a line, a series circuit of a resistor R2 and a capacitor C1 is connected to a cathode of the diode CD, and an end of the capacitor C1 is grounded a resistor R3 is connected in parallel to the capacitor C1, and an end of the resistor R3 is 1 grounded; a driving circuit 4b connected to an output end of the time constant circuit 4a, and a variable resistor 4c that is connected between an output end of the line resistor R1 and a ground and controls resistance of a variable resistance element VR by an output voltage of the driving circuit 4b. 
An output signal output from the emphasizer 3 is rectified and smoothed by the time constant circuit 4a in the above compression circuit 4, direct voltage according to a level of the input signal is supplied to the driving circuit 4b, the variable resistor 4c is driven by the driving circuit 4b, a level of the input signal of the line resistor R1 is changed, the input signal is compressed and supplied from an output end of the compression circuit 4 to a modulator 5, and a modulated signal is transmitted to space from the transmitting device 1 through a transmitting element 6 such as an infrared diode.
In the receiving device 2, the transmitted signal is received by a receiving element 7 such as a pin and a photodiode, demodulated by a demodulator 8, and supplied to a de-emphasis circuit 9 having a high-frequency down characteristic HL as shown in FIG. 10(B) that is a characteristic reverse to that of the emphasis circuit 3, and high frequencies of the transmitted signal are reduced in the de-emphasis circuit 9. In other words, an expanding operation is carried out and an analog acoustic signal is output to an output terminal T2 to which a headphone or the like is connected.
According to the transmitting/receiving device with the above structure, when the input level of the input signal increases as shown by an input/output characteristic in FIG. 11, the compression circuit 4 operates to control the level of the input signal so as to obtain a compression characteristic CC and such that an input level does not exceed a predetermined value as compared with a linear characteristic LC.
Therefore, it is possible to suppress apparent S/N degradation of a received signal due to a narrow dynamic range of the infrared wireless analog transmission line and to improve the dynamic range.
According to the above transmitting/receiving device and transmitting/receiving method of the acoustic signal, the time constant circuit 4a is necessary to detect the level of the input signal and a time delay is generated before the time constant circuit 4a operates to change a value of the variable resistor 4c into a value at a predetermined level through the driving circuit 4b. 
If a burst signal SB as shown in FIG. 12(A) is supplied as an input signal to the compression circuit 4, delays T1 and T2 corresponding to the time constant of the time constant circuit 4a are generated at a rising edge and a falling edge of the burst signal SB and time T1 is required to judge a correct level at the rising edge as shown in FIG. 12(B). If a frequency of the input signal is higher than the time constant of the time constant circuit 4a, a signal input during this period cannot be compressed and is output as it is.
FIG. 12(C) shows an output of the burst signal at this time. The transmitting device 1 is brought into an overmodulation state and a noise may be generated and the sounds may be interrupted in transmitted and received sounds at the rising edge 35 of the burst output signal SBO. Even if a signal in a specific frequency band is large, because every frequency band is attenuated simultaneously by the compression circuit 4, volume may be felt to be small only during operation of the compression circuit 4.
The present invention has been accomplished to solve the above problems and it is an object of the invention to provide a transmitting/receiving device and a transmitting/receiving method in which generation of noise and interruption of sound in transmitted/received sound are prevented and a feeling of reduction of a level is suppressed, the noise and interruption of the sound being generated when an acoustic signal is compressed and transmitted through a time constant circuit of a compression circuit, a part of the input signal cannot be compressed and output as it is, and the transmitting device is brought into an overmodulation state.